Thermal spraying, also known as flame spraying, involves the heat softening of a heat fusible material such as metal or ceramic, and propelling the softened material in particulate form against a surface which is to be coated. The heated particles strike the surface where they are quenched and bonded thereto. A conventional thermal spray gun is used for the purpose of both heating and propelling the particles. In one type of thermal spray gun, the heat fusible material is supplied to the gun in powder form. Such powders are typically comprised of small particles, e.g., between 100 mesh U.S. Standard screen size (149 microns) and about 2 microns.
A thermal spray gun normally utilizes a combustion or plasma flame to produce the heat for melting of the powder particles. Other heating means may be used as well, such as electric arcs, resistance heaters or induction heaters, and these may be used alone or in combination with other forms of heaters. In a powder-type combustion thermal spray gun, the carrier gas, which entrains and transports the powder, can be one of the combustion gases or an inert gas such as nitrogen, or it can be simply compressed air. In a plasma spray gun, the primary plasma gas is generally nitrogen or argon. Hydrogen or helium is usually added to the primary gas, and the carrier gas is generally the same as the primary plasma gas.
One form of powder for thermal spraying is composite or aggregated powder in which very fine particles are agglomerated into powder particles of suitable size. Such powder formed by spray drying is disclosed in U.S. Pat. No. 3,617,358 (Dittrich). This method is useful for producing powder having several constituents such as a metal and a ceramic. Agglomerated powder also may be made by blending a slurry of the fine powder constituents with a binder, and warming the mixture while continuing with the blending until a dried powder of the agglomerates is obtained. U.S. Pat. No. 4,645,716 (Harrington et al) teaches a homogeneous ceramic composition produced by this method. If one of the constituents is nearly the size of the final thermal spray powder, the composite is not homogeneous and, instead, comprises the larger core particles with the finer second constituent bonded thereto. Such a clad powder is disclosed in U.S. Pat. No. 3,655,425 (Longo et al).
The latter patent is particularly directed to a clad powder that is useful for producing thermal spray coatings that are abradable such as for clearance control applications in gas turbine engines. A constituent such as boron nitride is clad to nickel alloy core particles. The boron nitride is not meltable and so is carried into a coating by the meltable metal core in the thermal spray process. The patent teaches that the core is only partially clad in order to expose core metal to the heat of the thermal spray process. Optionally, fine aluminum is added to the cladding with improvements that are speculated in the patent to be related to an exothermic reaction between the aluminum and the core metal.
Another thermal spray powder in successful use for producing abradable coatings is sold by The Perkin-Elmer Corporation as Metco 313 powder. This is formed by cladding about 50% by weight of very fine powder of an aluminum alloy containing 12% silicon onto graphite core particles. Although this material has been well established for many years as a clearance control coating in turbine engines, for certain engine parts there has been a need for improved resistance to electrochemical reaction. Also there is always a need for improved abradability of clearance control coating without sacrificing resistance to gas and particle erosion.